Method of highly-densifying powdered metal

ABSTRACT

AN AIR-TIGHT CAN IS PREPARED FROM HEAT-RESISTANT MATERIAL, SUCH AS ORDINARY STEEL OR STAINLESS STEEL HAVING ON AT LEAST ONE END A TUBULAR STEM THROUGH WHICH THE CAN IS FILLED WITH THE POWDERED METAL TO BE COMPACTED. THE INTERIOR OF THE CAN IS THEN EXHAUSTED TO A HGIH VACUUM BY CONNECTING THE STEM TO A VACUUM PUMP, WHEREUPON THE STEM IS SEALED OFF AS BY WELDING WHILE MAINTAINING THE HIGH VACUUM. THE THUS POWDER-FILLED EVACUATED CAN IS THEN HEATED TO A HIGH TEMPERATURE WHICH IN ONE EXAMPLE WAS 2150* F. FOR ABOUT ONE HOUR. WHILE AT THAT HEAT IT IS PLACED IN THE DIE CAVITY OF A PRESS WITH THE STEM PROJECTING INTO A SUFFICIENTLY DEEP RECESS IN EITHER THE LOWER PUNCH OR IN THE UPPER PUNCH. THE UPPER PUNCH (AND IN A MODIFICATION ALSO THE LOWER PUNCH) IS MADE OF SMALLER DIAMETER THAN THE DIE CAVITY SO AS TO LEAVE AN ANNULAR CLEARANCE SPACE BETWEEN THE SIDE WALL OF THE DIE CAVITY AND THE PUNCH. THE UPPER PUNCH IS THEN LOWERED INTO THE DIE CAVITY IN A PRESSING STROKE WHICH FLATTENS THE CAN WHILE COMPRESSING THE METAL POWDER THERIN WITHOUT RUPTURING THE CAN OR LOWERING THE VACUUM THEREIN.   AS THE COMPRESSION OF THE CAN AND ITS CONTENTS CONTINUES, AND THE DENSITY OF THE POWDERED METAL MASS APPROACHES 100 PERCENT, THE SIDE WALLS OF THE CAN DO NOT COLLAPSE, BUCKLE OR CRINKLE, AS HAS HITHERTO OCCURRED IN PRIOR ATTEMPTS AT COMPRESSING POWDERED METALS WITHIN CANS. INSTEAD, THE PERIPHERAL PORTION OF THE CAN AND ITS CONTENTS DEFORM AXIALLY INTO THE CLEARANCE SPACE BETWEEN THE PLUNCH AND THE DIE CAVITY SIDE WALL. IF THE PRESSING IS CONTINUED UNTIL THE DENSITY EXCEEDS 100 PERCENT, THE SURPLUS METAL POWDER AND THE PERIPHERAL PORTION OF THE CAN CONTINUE TO MOVE AXIALLY INTO THE ABOVE-MENTIONED ELEARANCE SPACE, THEREBY PRODUCING AN AXIALLY-PROJECTING LIP EXTENDING AROUND THE PERIPHERY OF THE CAN AND CONTAINING METAL POWDER AT A SOMEWHAT LOWER DENSITY THAN THE METAL IN THE REMAINING PORTION OF THE CAN. THE WALLS OF THE CAN ARE THEN REMOVED BY MACHINING OR BY PICKLING, TOGETHER WITH THE LESS DENSE PERIPHERAL PORTION OF THE NOW SUBSTANTIALLY SOLIDIFIED METAL DISC OR &#34;PANCAKE.&#34;

Oct. 12, 1971 J. HALLER METHOD OF HIGHLY Filed Nov. 26. 1968 -DENSIFYING POWDERED METAL 2 Sheets-Sheet 1 mvemoa JOHN HALLER ATTO RN EYS Oct. 12, 1971 -J. HALLER 3,611,546

METHOD OF HIGHLY-DENSIFYING POWDERED METAL Filed Nov. 26, 1968 2 Sheets-Sheet 2 n will INVENTOR JOHN HALLER /72 ya ATTORNEYS United States Patent Oflice 3,611,546 Patented Oct. 12, 1971 3,611,546 METHOD OF HIGHLY-DENSIFYING POWDERED METAL John Haller, Northville, Mich., assignor to Federal- Mogul Corporation, Southfield, Mich. Filed Nov. 26, 1968, Ser. No. 779,151

Int. Cl. B2212 US. Cl. 29-420 14 Claims ABSTRACT OF THE DISCLOSURE An air-tight can is prepared from heat-resistant material, such as ordinary steel or stainless steel having on at least one end a tubular stern through which the can is filled with the powdered metal to be compacted. The interior of the can is then exhausted to a high vacuum by connecting the stem to a vacuum pump, whereupon the stem is sealed oif as by welding while maintaining the high vacuum. The thus powder-filled evacuated can is then heated to a high temperature which in one example was 2150 F. for about one hour. While at that heat it is placed in the die cavity of a press with the stern projecting into a sufiiciently deep recess in either the lower punch or in the upper punch. The upper punch (and in a modification also the lower punch) is made of smaller diameter than the die cavity so as to leave an annular clearance space between the side wall of the die cavity and the punch. The upper punch is then lowered into the die cavity in a pressing stroke which flattens the can while compressing the metal powder therein without rupturing the can or lowering the vacuum therein.

As the compression of the can and its contents continues, and the density of the powdered metal mass approaches 100 percent, the side walls of the can do not collapse, buckle or crinkle, as has hitherto occurred in prior attempts at compressing powdered metals within cans. Instead, the peripheral portion of the can and its contents deform axially into the clearance space between the punch and the die cavity side wall. If the pressing is continued until the density exceeds 100 percent, the surplus metal powder and the peripheral portion of the can continue to move axially into the above-mentioned clearance space, thereby producing an axially-projecting lip extending around the periphery of the can and containing metal powder at a somewhat lower density than the metal in the remaining portion of the can. The walls of the can are then removed by machining or by pickling, together with the less dense peripheral portion of the now substantially solidified metal disc or pancake.

BACKGROUND OF THE INVENTION So-called super-alloys primarily containing cobalt or nickel bases have been widely used in aerospace technology because of their capability of retaining their tensile strengths at higher temperatures than ordinary alloys, as well as possessing superior corrosion resistance and less creep or deformation by metal flow at such temperatures. Such alloys have hitherto been available solely in cast form and have not only contained too many impurities which are difiicult to eliminate but have also lacked uniform metallurgical characteristics, especially for meeting the extremely exacting requirements of aerospace apparatus. When, however, such alloys have been prepared in powdered form to produce a wrought product and then compacted to almost solid densities, the resulting billets or bodies have been found to possess a better metallurgical structure with more uniformity throughout the billet or body and at the same time are free from impurities. Furthermore, such billets are free from the voids, blow holes or pockets of cast billets and, unlike cast billets, may be further forged or worked to obtain additional benefits.

Hitherto, powders of such metals have been compacted to densities which approach the percent density of solid metals by placing the loose powder in an elongated metal can which is then evacuated, heated to a high temperature and then compressed in a die cavity. While this technique maintains cleanliness of the metal powder and prevents oxide formation so that the powder particles fuse cleanly during hot pressing, nevertheless the side walls of the can buckle and crinkle during such pressing. When the can is removed by machining or pickling, the peripheral portion of the metal body must also be removed, with consequent loss of material. The present invention prevents such crinkling and greatly reduces the amount of metal which is lost at the periphery of the metal body.

Experience hitherto with cast super-alloy billets has shown that the high temperatures necessary for bringing the billet to the desired shape result in a coarse-grained metallurgical structure which in turn is undesirable, and the cast billets are not capable of being reforged or reworked. Ordinarily, it is found that the higher the temperature used in forging, the coarser the metallurgical grain structure which is obtained. In contrast to these defects of cast super-alloy billets, the present invention produces a metallurgical structure which is extremely fine grain and the billets produced by this powder metallurgy process not only possess a more uniform grain structure but also can be reforged or reworked which cannot be done with cast super-alloys.

In the drawings, FIG. 1 is a central vertical cross-section through a container while it is being filled with metal powder according to an early stage in the method of the present invention;

FIG. 2 is a view similar to FIG. 1 after the container has been filled with metal powder, evacuated and sealed, and then placed in the cavity of the die, ready for com pression;

FIG. 3 is a View similar to FIG. 2 but with the die cushion omitted and showing the relative positions of the die, the container and the upper and lower punches after pressing has been completed;

FIG. 4 is a view similar to FIG. 3, but showing a modification for compressing an axially thicker container than the container shown in FIGS. 1, 2 and 3, whereby a clearance space is provided both between the upper and lower punches and the side walls of the die cavity to permit axial deformation of the container and its contents in opposite directions;

FIG. 5 is a central vertical section through a further modification employing an annular container while it is being filled With metal powder through a plurality of hollow stems;

FIG. 6 is a view similar to FIG. 5 but with the filled and sealed container inverted within the cavity of a die, ready for compression;

FIG. 7 is a view similar to FIG. 6, but showing the relative positions of the die, the container, the core and the punch after pressing has been completed; and

FIG. 8 is a view similar to FIG. 7 but showing a still further modification for compressing an axially thicker container than those of FIGS. 1, 2 and 3, in that a clearance space is provided not only between the upper punch and the die cavity but also between the lower punch and the die cavity walls so as to permit axial deformation of the thicker annular container and its contents in opposite directions.

Referring to the drawings in detail, FIG. 1 shows a powder-filled container assembly, generally designated 10, consisting of a container 12 in the course of being filled with a charge 14 of metal powder to be compacted to a high density approaching or equalling solidity or 100 percent density. The container 12 may be either of ordinary steel or of stainless steel, and has a cylindrical side Wall 16 and substantially flat top and bottom walls 18 and 20 respectively. The top wall 18 is provided with a hole 22 over which a tubular filling and evacuating stem 24 is welded or otherwise secured. The stem 24 is provided with a filling passageway 26 leading to the chamber 28 within the container 12.

The die set 30 in which the container assembly 10 is to be compressed is shown diagrammatically in FIGS. 2 and 3 and consists of a die 32 into the die bore 34 of which an upper punch 36 and a lower punch 38 enter from opposite directions. The die 32 is preferably yieldingly supported on the bed or bolster 40 of a conventional hydraulic or mechanical press (not shown), as by by a die cushion 42 consisting of compression springs 44 or of a conventional hydraulic die cushion (not shown). Such die cushions are well known in the press art and their details are beyond the scope of the present invention. The lower punch 38 is preferably movably connected to a lower press plunger (not shown) to eject the finished workpiece, as is well known in the press art. In the die set 30. shown in FIGS. 2 and 3, the upper and lower punches 36 and 38 have side walls 46 and 48. The side wall 46 is of smaller diameter than the die bore 34 so that a comparatively wide clearance space 50 occurs therebetween. The lower punch 38 in FIGS. 2 and 3, however, has its side wall 48 snugly but slidably fitting the die bore 34. The upper and lower punches have bottom and top surfaces 52 and 54 respectively, the latter containing a recess 56 for the reception of the stem 24.

In carrying out the method as shown in FIGS. 1, 2 and 3, the container 12 prepared as shown in FIG. 1 and described above, is completely filled with the charge 14 of powdered metal, which may be of the so-called superalloy type containing cobalt or nickel for resisting deformation and retaining high tensile strength at a high temperature. After the chamber 28 has been completely filled through the stem 24, the latter is connected to a high vacuum pump (not shown) and the chamber 28 evacuated as completely as is commercially possible to prevent subsequent oxidation of the particles of metal powder in the charge 14 thereof. While the chamber 28 is still being kept in a highly evacuated condition, the stem 24 is sealed off, as by welding, to produce a sealed air-tight tip 58.

The thus-evacuated and sealed container and powder assembly 10 is heated in a suitable furnace to a temperature of approximately 2150 F. for approximately one hour, and then transferred to the die cavity formed by the die bore 34 and the top surface 54 of the lower punch 38 (FIG. 2), with the stem 24 projecting downward into the recess 56. The upper punch 36 is then caused to descend into the die cavity 60 (FIG. 3), compressing the container and powder assembly 10 between its lower surface 52, the upper surface 54 of the lower punch 58 and the die bore 34. As this hot pressing operation proceeds, densification of the powdered metal charge 14 progressively increases as the upper punch 36 descends and the powdered metal particles are forced into closer and closer engagement until fusion occurs between them by the combined action of the initial high temperature and the added heat resulting from compressive force exerted.

When the density of the powdered metal charge 14 exceeds 90 percent, the powdered metal charge 14 becomes denser in its central portion than at its periphery and when density approaches 100 percent or complete solidity, the peripheral portion of the container 12 becomes deformed axially by being extruded into the clearance space 50 between the upper punch side wall 46 and the die bore 34 forming an axially-projecting annular rib or lip 62.

If the density of the charge 14 comes to exceed 100 percent, the excess powdered metal is likewise extruded into the annular groove or channel 64 within the annular lip 62, and is of lower density than the remaining portion of the now virtually solidified charge 14. The upper punch 36 is then retracted out of the die cavity 60 and the nowcompressed container and powdered metal assembly 10 ejected from the die cavity 60, such as by moving the lower punch 38 upward until its top surface 54 reaches the same level as the top surface 66 of the die 32. The container 12 is now removed by machining or by other suitable means, together with the less dense peripheral portion of the now-solidified charge 14, leaving the now solidified latter charge 14 as a pancake 68 of substantially solid metal.

In the modified die set 70 of FIG. 4, the upper punch 36, die 30 and lower punch 38 remain substantially the same as in FIGS. 2 and 3, together with the die cushion 42, except that an annular rabbet 72 encircles the top of the lower punch 38 and provides an annular clearance space 74 corresponding to and axially opposite the clearance space 50. The die set 70 of FIG. 4 is employed in place of the die set 30 of FIGS. 2 and 3 where it is desired to compress a container and metal powder assembly 76 which is axially thicker than the assembly 10 of FIG. 1. Filling, evacuation, sealing, heating and loading into the die cavity 60 occur as described above. During the compression stroke of the press, the assembly 76 is compressed between the upper and lower punches 36 and 38 as before. However, as the density of the charge 78 therein approaches 100 percent, the peripheral portions of the container 80 and the metal powder contained therein are extruded axially in opposite directions to provide upper and lower annular lips 82 and 84 extending into the clearance spaces 50 and 74 with the surface metal powder flowing into the peripheral channels 86 and 88 within the annular lips 82 and 84.

The upper punch 36 is then retracted into its raised position and the lower punch 38 caused to move upward, as before, to eject the assembly 76. The container 80 is then removed by machining or any other suitable manner, together with the peripheral portion of the now solidified metal charge 78 forming, as before, a metal pancake or billet which is, however, axially thicker than the pancake of billet 68 formed during the procedure set forth in FIGS. 1 to 3 inclusive.

The further modified powder-filled container assembly, generally designated 90, shown in FIGS. 5, 6 and 7, is of annular shape and consists of a flat relatively shallow annular container 92. As before, the container 92 is preferably formed of a high temperature oxidation-resistant metal, such as stainless steel, and has outer and inner cylindrical side walls 94 and 96 respectively whose height is likewise a fractional part of the diameter of the container 92 and which are interconnected by annular top and bottom walls 98 and 100 respectively with a central opening 102 therethrough. The top wall 98 is provided with one or more holes 104 over which one or more tubular filling and evacuating stems 106 are welded or otherwise secured. The filling stems 106 are provided with filling passageways 108 leading to an annular chamber 110 within the container 92 for receiving the powdered metal charge 111.

The die set 112 in which the container and powder assembly 90 is to be compressed (FIGS. 6 and 7) consists of a die 114 having a die bore 116 into the upper and lower ends of which an upper punch 118 and lower punch 120 enter from opposite directions. The die 114 is preferably yieldingly supported on the bed or bolster 122 of a conventional hydraulic or mechanical press (not shown) by the die cushion 124 shown in FIG. 6 as consisting of a plurality of compression springs 126 arranged at intervals around the periphery of the die 114. Cooperating with the die 114 and upper and lower punches 118 and 120 is a central core 128 which occupies the opening 102 in the assembly 90 during the carryingout of the method pertaining to FIGS. 5, 6 and 7.

The upper and lower punches 118 and 120 have side walls 130 and 132 respectively. The side wall 130 of the upper punch 118 is of smaller diameter than the die bore 116 (FIG. 7) to provide an annular clearance space 134 therebetween. The side wall 132 of the lower punch 120 snugly but slidably fits the die bore 116. The central portion of the upper punch 118 is provided with a recess 136 which at its lower end is provided with a counterbore 138 creating an annular clearance space 140 between the side wall 142 of the core 128 and the counterbore 138. The upper punch 130 is provided with an annular lower end surface 144 forming a pressing surface, whereas the lower punch 120 is provided with a flat upper end surface 146 forming an abutment surface, the central portion of which is recessed at 148 to receive a central boss 150 which projects downward from the core 128. In carrying out the method as shown in FIGS. 5, 6 and 7, the container 92 is filled with the charge 111 of powdered metal through the filling passageway or passageways 108 in the stem or stems 106 in the manner described above in connection with FIGS. 1, 2 and 3, after which the chamber 110 is evacuated by connecting the stem or stems 106 to a high vacuum pump, whereupon the stems 106 are sealed off, as by welding, to produce one or more sealed air-tight tips 152. The thus-evacuated and sealed container and metal powder assembly 90 is then heated in a suitable furnace and transferred to the die cavity 154 formed by the die bore 116 and the top surface 146 of the lower punch 120, with the sealed stems 106 projecting downward into recesses 155 in the top surface 146. The upper punch 118 is then caused to descend into the die cavity 154 (FIG. 7), compressing the container and powder assembly 90 between the lower surface 144 of the upper punch 118 and the upper surface 146 of the lower punch 120 while the wall of the die bore 116 prevents lateral expansion. As this hot-pressing operation proceeds, densification of the powdered metal charge 111 progressively increases as the upper punch 118 descends and the powdered metal particles in the container 92 are compressed. These particles in the charge 111 are progressively forced into closer and closer engagement with one another until fusion occurs between them by the combined action of the initial high temperature and the added heat resulting from the compressive force exerted.

As before, when the density of the powdered metal charge 111 approaches 100 percent or complete solidity, the major portions of the container 92 between the outer and inner side walls 94 and 9-6 are compacted axially while the inner and outer peripheral portions thereof adjacent the side walls 96 and 94 become deformed axially (FIG. 7) by being extruded into the clearance spaces 134 and 140 respectively, thereby forming outer and inner hollow annular ribs or lips 156 and 158 containing less densely compacted portions of the powdered metal charge 111. If the compression is carried to a point where the density of the compressed charge 111 exceeds 100 percent, the excess powdered metal is likewise extruded into the annular grooves or channels 160 and 162 within the ribs or lips 156 and 158, leaving the remaining portion of the charge 111 virtually solidified.

When compression of the charge 111 has been completed, the upper punch 118 is retracted out of the die cavity 154 and the now-compressed or compacted container and powdered metal assembly 90 is ejected from the die cavity 154, such as by moving the lower punch 120 upward until its top surface 146 reaches the same level as the top surface 164 of the die 114. The container 92 is now removed by machining or other suitable means, together with the less dense peripheral portions of the now-solidified charge 111, leaving the latter as a doughnut or annular block 168 of substantially solid metal.

In the modified die set- 170 shown in FIG. 8, the upper punch 118 and lower punch 120,remain substantially the same as in FIGS. 6 and 7, together with the die cushion 124 and core 128, with the exception of the fact that an annular lower clearance space 172 is now provided between the reduced diameter side wall 174 of the lower punch 120 and the die bore 116. As in FIG. 4, the die set 170 is employed in place of the die set 112 when it is desired to compress a container and metal powder assembly 176 which is axially thicker than the assembly of FIG. 5. Filling, evacuation, sealing, heating and loading onto the die bore 116 occur as described immediately above. During the compression stroke of the press, the assembly 176 is compressed between the upper and lower punches 118 and 129 as before. However, as the density of the charge 178 therein approaches percent or complete solidity, the outer and inner peripheral portions of the annular container 180 and the metal powder contained therein are extruded axially in opposite directions to provide outer and inner upper lips or ribs 182 and 184 respectively as before, and in addition, outer and inner lower lips or ribs 186 and 188 respectively. The former expand into the upper outer and inner clearance spaces 190 and 192 provided between the die bore 116, the upper punch 118 and the core 128, whereas the latter expand into the lower outer and inner clearance spaces 172 and 194 provided between the die bore 116, the lower punch and the core 128, which in turn extends into the counterbore 196 containing a central recess 198 into which the boss fits. The excess powdered metal, if any, in the charge 178 also flows into the ribs or lips 182, 184, 18-6 and 188.

The upper punch 118 is then retracted into its raised position and the lower punch 120 again caused to move upward to eject the now compressed assembly 176. The container is then removed by machining or other suitable means, together with the peripheral portion of the not solidified charge 178 so as to form an axially thicker solid metal doughnut or annulus.

The foregoing example, wherein the powder-filled can was heated to 2150 F. for one hour was satisfactory for the particular size of the billet to be produced. A range of temperatures from 2000" F. to 2200 F. is satisfactory for the range of sizes to be produced whereas the period of one hour is increased with an increase of size and weight as found suitable for the particular job. If an ordinary steel can is used rather than one of stainless steel, the sheet metal forming the walls of the can can be peeled oil the compacted metal billet after the compacting procedure has been carried out, thus making its removal quicker as well as simpler and less expensive than by machining it off.

I claim:

1. A method of making a substantially solid body from powdered metal, comprising encasing the powdered metal in a deformable metal container, creating a vacuum within said container, heating the thus evacuated container and its contents to a temperature suflicient to facilitate subsequent deformation of said container and compaction and coalescence of the powdered metal particles,

applying axial compressive force to said container sufficient to deform said container axially and to compact the powdered metal therein into a substantailly solid body,

restraining said container from expanding laterally during the application of said axial compressive force while permitting a peripheral portion of said container and the powdered metal therein to deform axially,

terminating the application of said compressive force,

and removing the thus deformed container and the peripherally-deformed portion of said powdered metal from said body.

2. A method, according to claim 1, wherein the axial peripheral deformtion is effected in the opposite axial direction to the direction of application of the axial force.

3. A method, according to claim 1, wherein the axial peripheral deformation is efiected in the same axial direction as the direction of application of the axial force.

4. A method, according to claim 1, wherein the axial deformation is effected oppositely in both axial directions.

5. A method, according to claim 1, wherein the heating of the container and its contents is carried out to temperature of approximately 2150 F. for approximately one hour prior to applying said compressive force.

6. A method, according to claim 1, wherein the container is of approximately hollow cylindrical shape.

7. A method, according to claim 6, wherein the container has a substantially flat top wall and a substantially flat bottom wall, and wherein said axial force is exerted against one of said fiat walls.

8. A method, according to claim 1, wherein the container is of approximately annular shape with approximately cylindrical outer and inner walls.

9. A method, according to claim 8, wherein the axial peripheral deformation is effected adjacent said outer and inner walls.

10. A method, according to claim 9, wherein said deformation is also effected in the outer and inner peripheral portions of both said top and said bottom wall.

11. A method, according to claim 1, wherein said container is confined within a die cavity during the application of said axial compressive force.

12. A method, according to claim 11, wherein said compressive force is applied by a pressing punch having a clearance space separating it laterally from said die cavity, and wherein the container is caused to expand and deform into said clearance space.

13. A method, according to claim 8, wherein said container is confined within a die cavity between a core and the side wall of said cavity, wherein said compressive force is applied by a pressing punch with an annular pressing portion having annular outer and inner clearance spaces separating it from the side wall of said die cavity and from said core, and wherein the container is caused to expand and deform into said clearance spaces.

14. A method, according to claim 1, wherein said body is compacted to a density of at least 95 percent.

References Cited UNITED STATES PATENTS 3,075,244 l/1963 Glenn 29-420 X 3,269,826 8/1966 Bumgarner 29420 X 3,496,036 2/1970 Di Giambattista 75-214 X JOHN F. CAMPBELL, Primary Examiner D. C. REILEY, Assistant Examiner US. Cl. X.R. 29 423; 75 214 

